I think OS 3.9 includes workbench.library
When installing it, you should do it by installing over a fresh install of A4000T WB 3.1 anyway.
The OS 3.9 Boing Bag 1 has a bug that caused the machine not to boot (I think it was a SCSI controller conflict).
I reported this and it was fixed in Boing Bag 2.
